Multihoming is an increasingly popular approach for enhancing Internet connectivity. While commonly associated with large routers and complex BGP configurations, the availability benefits of multihoming can be achieved at low cost using relatively simple techniques. But your multihoming strategy must be carefully planned to ensure that you actually improve the Internet availability seen by your company.

This article is adapted from the author's book High Availability Networking with Cisco (Addison-Wesley, 2001, ISBN 0-201-70455-2).

Many organizations depend on Internet connectivity to support critical applications.
One popular approach for improving Internet connectivity is to connect to more
than one Internet service provider (ISP), a technique called multihoming.

Multihoming can be very effective for ensuring continuous connectivity—eliminating
the ISP as a single point of failure—and it can be cost-effective as well.
However, your multihoming strategy must be carefully planned to ensure that
you actually improve connectivity for your company.

The Need for Physical Diversity

Taking advantage of redundant links requires three conditions:

You must be able to detect when a link has failed.

You must have a mechanism for directing traffic that would normally flow
across a failed link to take the path that's still functional.

Meeting the first two conditions only helps to the extent that whatever
causes the failure of one link doesn't also result in failure of its
backup.

Let's look at the last requirement first; no protocol design will save
you if all links have failed. Because most network failures are due to problems
in the WAN links, it does little good to connect to a second ISP if both ISP
links are carried over the same communications circuit. Even if independent
circuits are used, if they're not physically diverse they'll still be
subject to common failure events such as construction work inside your building
or digging in the street outside. This independence ultimately needs to extend
to the physical environment in the data center, where the routers and interfaces
should have independent sources of power and be physically separated so that an
accident affecting one link won't affect the devices supporting the
other.

Providing complete physical diversity is difficult and expensive, and the
requirement is not limited to ISP connections. All critical network links for
internal communications should also be diversified. Assuming a well designed
internal network, the easiest way to achieve physical diversity in your ISP
connections is to connect from two different locations that are already well
connected to each other, but far enough apart to not share any common facilities
or communications infrastructure.